Photoelectric scanners are widely used in industrial control systems for measurement or detection of objects passing through a light beam passing between emitter and receiver portions of the scanner system. Most scanner systems use light emitting diodes (LED's) as the light emitting element because they have a number of advantageous electrical and optical properties. Virtually all LED scanner systems use a modulated or pulsed mode of operation for the LED, as opposed to continuous operation. Pulsed operation permits the receiver to be AC coupled so that it will be immune to ambient or steady state light, and it also permits operating the LED at a high peak power with a low duty cycle in order to keep the average power within the limit for the particular LED. The higher peak power means that the scanner can be operated at a longer range, i.e. with a greater optical path length between emitter and receiver.
In modulated LED scanner systems the choice of modulation or pulse frequency is a compromise between conflicting requirements for response time and range. The reason for this is that most receivers are designed either to integrate or count pulses received, and to respond only to the receipt of a certain number of consecutive pulses. This operation is used to provide noise immunity to avoid false operation due to high frequency light sources such as fluorescent and vapor lamps or other photoelectric controls, or due to electrical interference. Since a specific number of pulses must be received successively in order for the control to provide a true change in output signal, it is apparent that the response time is inversely proportional to the frequency of operation, and that a high frequency would be desirable to minimize response time. However, due to requirements of the light receiving circuitry, it is generally necessary that the duration of the individual emitted light pulses be held to at least some minimum value, for example 10 microseconds. Since this duration cannot be reduced as the repetition rate is increased, the result is an increase in the duty cycle for the LED, which means that the LED would be operated at higher average power levels with increasing frequency. It is therefore necessary to select the peak power during a pulse of the LED in conjunction with the duty cycle to keep average power within safe limits for the particular LED. Thus a trade-off is possible between fast response time and shorter range, or slow response time and longer range. Because of this trade-off some manufacturers have provided several models of each type of scanner designed to different response time and power ranges to meet different needs for different applications. However, that involves additional cost in manufacturing and maintaining a larger inventory of parts, both for the manufacturer and for large users who have to stock a number of different models for different applications.
It would be possible to provide a switch or potentiometer on the scanner to adjust the frequency of operation and thereby adjust the response time. However, that would require that the power to the LED be set for the worse case (highest frequency) and therefore would provide no benefit regarding maximizing range. To maximize range it would be necessary to adjust the power to the LED as the adjustment to operating frequency were made. This oould be done by providing switches or matched potentiometers that would switch or change component values in both the frequency determining circuits and the LED power determining circuits, but these solutions are costly, both in terms of cost of manufacture and in terms of requirement for space in the device, many of which are miniaturized for flexibility of applications. In the case of switches, there is a further disadvantage of not providing continuously variable parameters for adjustment to specific needs. If matched potentiometers were used, they would have to be matched to a high degree of precision so that the LED power could be safely adjusted to optimum corresponding to each change in frequency, and the high precision would add to cost of manufacture.